Background: Previous studies have suggested that patients with esophageal squamous cell carcinoma (ESCC) are still at a high risk of developing second primary malignancies (SPMs) after definitive therapies. We evaluated the development of SPMs and explored its risk factors in patients with clinical T1bN0 ESCC. Methods: JCOG0502 prospectively compared esophagectomy with definitive chemo-radiotherapy for clinical T1bN0 ESCC. Here, we reviewed all JCOG0502 patients’ data for SPMs and investigated the risk factors for SPMs using uni-variable and multivariable analyses by Fine and Gray model. Results: Among 379 enrolled patients, 213 underwent esophagectomy and 166 received chemo-radiotherapy. Patient characteristics were male (85%); median age [63 (range 41–75) years; location of the primary tumor (upper/middle/lower thoracic esophagus, 11%/63%/27%, respectively]; alcohol consumption history (79%); smoking history (66%); prevalence of no/several/many/unknown Lugol-voiding lesions (LVLs) (45%/36%/8%/11%, respectively). In a median follow-up of 7.1 years, 118 SPMs occurred in 99 (26%) patients. Cumulative incidences of SPMs after 3, 5, and 10 years were 9%, 15%, and 36%, respectively. The most common primary tumor sites were the head and neck (35%), stomach (20%) and lungs (14%). In multivariable analyses, compared to no LVLs, several LVLs [hazard ratio (HR) 2.24, 95% confidential interval (CI) 1.32–3.81] and many LVLs (HR 2.88, 95% CI 1.27–6.52) were significantly associated with the development of SPMs. Sixteen patients died due to the SPMs. Conclusion: The incidence of SPMs was high. The presence of LVLs, which was a predictive factor for SPMs, may be useful for surveillance planning.
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